The present invention relates to an image forming apparatus such as an electrophotographic copying machine or the like, and more particularly to an image forming apparatus employing a novel separating method, wherein a recording sheet (usually a copy paper) to which a toner image has been transferred from an image carrying member on which the toner image was first developed by developing process, is then separated from the image carrying member by a separator having an A.C. component.
In the conventional image forming apparatus of the aforesaid type, both the transfer unit and the separating unit employ a corotron corona discharging device wherein a transfer electrode and a separation electrode in each, both comprised of a wire, or wires stretched over a photoreceptor as an image carrying member with a slight distance from the photoreceptor, conduct corona discharge. The function of the separating unit is to discharge by means of an A.C. corona discharging, the charge on a transfer material having thereon a toner image transferred thereto by means of a transfer unit and being attracted electrostatically to the surface of the image carrying member, and thereby to cause the recording sheet to be separated from the drum-shaped image carrying member. A separating function required for the separating unit is not only to cause the recording sheet to be separated from the image carrying member but also to cause the recording sheet to be separated with a perfect toner image thereon. Namely, even a part of a toner image on the recording sheet should not be transferred again to the image carrying member.
It is not easy to cause the transfer to satisfy such an exact separating function.
Japanese Patent Examined Publication No. 17495/1978 and Japanese Patent Publication Open to Public Inspection Nos. 89430/1978 and 106233/1979 have been suggested from the aforesaid viewpoint, and both of them are related to an idea wherein the aforesaid separating function is achieved by changing the conditions for separation between the leading edge and the remaining portion of a copy paper.
As a method of changing the value of the separation current on the half way of separating a copy paper from an image carrying member, the following have been suggested.
(a) To change the intensity of A.C. corona discharge.
(b) To change the value of a D.C. bias superposed on A.C.
(c) To use D.C. for neutralizing the charge at the leading edge of a copy paper and use A.C. for neutralizing the other portion thereof.
However, these ideas have the following disadvantages.
(a) Two or more power supply outputs are necessary, which is expensive.
(b) It is difficult to change the conditions for separation stepwise or continuously.
In the meantime, there has been proposed a method wherein only one power supply is used and only the charge at the leading edge of a copy paper is neutralized by cutting the power supply in the middle of separation of the copy paper. In this method, however, the separation efficiency in the transfer is deteriorated.
An object of the invention is to provide an image forming apparatus having a separating function which prevents toners once transferred onto a recording sheet from being re-transferred back to the photoreceptor, while satisfying the separation requirements for a recording sheet including its leading edge, by maneuvering the separation current.
In this case, the object is to provide an image forming apparatus, preferably having only one power supply for neutralization for separation for a cost saving, and having satisfactory separation caused by changing the conditions for separation stepwise or continuously controlled by a CPU that is provided on the main body of the image forming apparatus.